Some test and measurement instruments, such as RSA6100 Series Real-Time Spectrum Analyzers and MDO4000 Series Mixed Domain Oscilloscopes, available from Tektronix, Inc. of Beaverton, Oreg., are capable of displaying both time-domain and frequency-domain views of an input signal.
When using such a test and measurement instrument, a user can sometimes encounter signals which are visible in the time-domain display but not in the frequency-domain display. This typically occurs because the time-domain display shows data from the entire acquisition bandwidth of the instrument, whereas the frequency-domain display only shows a subset of that data. That is because the frequency-domain display does not show the very ends of the acquisition bandwidth, which are distorted by attenuation and phase nonlinearity due to various analog and digital filters within the acquisition path of the instrument. As a result, a signal that is located within the acquisition bandwidth of the instrument but outside of the displayed span, i.e., a signal that is “in-band but out-of-span,” or simply, “out-of-span,” will be visible in the time-domain display but not in the frequency-domain display.
This discrepancy can result in a variety of confusing scenarios for a user.
One scenario is where time-domain and frequency-domain measurements simply do not agree. The user is left trying to figure out why these two supposedly equivalent measurements yield different results.
Another scenario is when excessive in-band but out-of-span power causes an overflow condition. A user looking in the frequency-domain display will not see the excessive power, and is left wondering why the instrument is showing an overflow condition.
Some spectrum analyzers such as the FSW Signal and Spectrum Analyzer available from Rohde & Schwarz of Munich, Germany allow the user to view an enlarged spectral display which shows the entire acquisition bandwidth of the instrument, including the band-edge roll-off. However, the problem with this implementation is that the user must be knowledgeable enough to know when to activate the enlarged spectral display. For users who are not experienced with spectrum analyzers, it can be very confusing to get either differing measurement results or unexplained warning messages. Without a significant amount of experience, the user may not know he or she should make the span setting larger to see the real problem.